Solid state imagers, including charge coupled devices (CCD) and CMOS imagers, have been used in photo imaging applications. A solid state imager circuit includes a focal plane array of pixel cells, each one of the cells including a photosensor, which may be a photogate, photoconductor or a photodiode having a doped region for accumulating photo-generated charge. Microlenses are placed over imager pixel cells to focus light onto the initial charge accumulation region of the photosensor.
In conventional imager devices, incoming photons of light en route to the photosensitive parts of the pixel pass through a color filter array (CFA) used to obtain color signals for the inherently monochrome image device. To get color signals out of an image device requires pixel cells which separately detect red (R), green (G), and blue (B) photons of received light. To do this, each pixel of the imager device is covered with either a red, green, or blue filter, according to a specific pattern. A conventional color pattern for a color filter array used in today's imager devices, known as the Bayer pattern, is shown in FIG. 1.
As shown in a top-down view of FIG. 1, the Bayer pattern includes a 2-by-2 set 25 of pixels arranged in a repeating pattern over the entire pixel array 50. Every other row consists of alternating Red (R) and green (G) colored pixels cells, while the other row consists of alternating Green (G) and blue (B) color pixels. In conventional imager devices, one microlens 30 is formed over each individual pixel 20 as shown in FIG. 1. When an image sensor utilizing a Bayer pattern color filter array is read out, line by line, the pixel sequence comes out RGRGR, etc., and then the alternate line sequence is GBGBG, etc. This output is called sequential RGB (or sRGB).
Use of microlenses significantly improves the photosensitivity of the imager device by collecting light from a large light collecting area and focusing it onto a small photosensitive area of the photosensor. However, as the size of imager arrays and photosensitive regions of pixels continue to decrease, due to desired scaling, it becomes increasingly difficult to provide a microlens capable of efficiently focusing incident light rays onto the photosensitive regions of the pixel cell. Both the optical and electrical performance of the imager device may suffer with lens scaling. This problem is due in part to the increased difficulty in constructing a microlens that has the optimal focal characteristics for the increasingly smaller imager device. Beyond an optimal size for a microlens, further scaling down of the microlens causes the light gathering power of the lenses to drop off significantly.
Scaled microlenses may have a negative impact on both the internal and external quantum efficiency of imager devices using the smaller microlenses. Microlenses are diffraction limited, as the angular resolution of every microlens is inversely proportional to the diameter of the lens. The diffraction limit fundamentally affects the quality of an image reproduced by an imager device. Accordingly, it is desirable to form a microlens array that can be used with scaled pixels without suffering the drawbacks of conventional, scaled microlenses.